This invention relates generally to torsional oscillators adapted to produce a vibratory motion of high amplitude, and more particularly to a control system for a torsional optical scanner having a relatively broad mechanical bandwidth, said system acting to maintain the scan at a substantially constant amplitude.
Various forms of optical devices are currently in use to chop, scan, sweep or otherwise deflect or modulate a light or other beam of radiant energy. Such optical devices are incorporated in mass spectometers, bolometers, corimeters, horizon sensors and in various instruments which utilize or analyze nuclear, X-ray or laser beams, or beams in the visible, ultraviolet or infra-red region. In recent years, the need has arisen for optical scanners adapted to sweep a light beam across binary-coded bars or similar indicia to produce signal pulses for carrying out various computerized functions.
Existing optical devices for these purposes usually make use of motor-driven discs, drums, mirrors or prisms that are relatively cumbersome and have large power requirements. Also in use is an electromechanically-actuated armature device in which a pivoted armature carrying an optical element is mounted in jewel bearings. Such optical modulators are relatively inefficient and unstable and also lack shock resistance.
The drawbacks characteristic of existing mechanical oscillators are overcome in the resonant torsional oscillator disclosed in the patent to Dostal U.S. Pat. No. (3,609,485), wherein an erect torsional rod, anchored at its base, is electromagnetically-driven at a point adjacent the base to cause an optical element secured to the free end of the rod to swing back and forth at a rate determined by the resonance frequency of the oscillator.
Inasmuch as the ratio of the angular swing at the free end of the rod with respect to the angular swing at the drive point depends on the distance between the drive point and the free end of the rod relative to the shorter distance between the drive point and the base of the rod, the deflection of the optical element at the free end is greatly amplified.
With an arrangement of the Dostal type, certain practical difficulties are experienced when the drive pulses for actuating the rod are derived from an external power source. In the case where the repetition rate of the power source signal is highly stable and corresponds to the natural resonance frequency of the torsion rod, the resultant amplitude of the swing at the free end of the rod is normally high, but is adversely affected by changes in ambient temperature which alter the resonance frequency of the rod.
A torsion rod is a mechanical resonator whose natural frequency is determined by its dimensions and Young's modulus of elasticity. The mechanical bandwidth or response-curve of a torsion rod depends on its Q or quality factor--the higher the Q, the narrower or sharper its bandwidth. That is to say, when a torsion rod oscillator has a high Q, it responds efficiently to drive pulses whose repetition rate exactly corresponds to its natural frequency. However, the response drops sharply as the natural frequency deviates from the frequency of the drive pulses or as the frequency of the drive pulses deviates from the natural frequency of the oscillator.
If, therefore, we have a stable source of drive pulses and the natural frequency of the high-Q mechanical oscillator is caused to depart from its assigned value by reason of temperature changes affecting the torsion rod dimensions, then the amplitude of rod oscillations will markedly decrease. On the other hand, should the source of drive pulses be unregulated, as a consequence of which the repetition rate deviates somewhat from the assigned oscillator frequency, then the amplitude of rod vibrations will decrease sharply to an extent determined by the degree of deviation.
Hence while a highly selective torsion rod oscillator of the type disclosed in the Dostal patent affords increased angular sensitivity, when this oscillator is driven from an external power source whose frequency is subject to variation or when the mechanical oscillator frequency is subject to change as a result of temperature changes, the oscillator in either case does not function efficiently.